Polarization mode dispersion (PMD) has become one of the most important limiting factors for high-speed optical communication systems, especially for existing optical networks. The existing optical fiber networks have poor PMD characteristics with the PMD ranging from 0.5 to 2 ps/km.sup.1/2, corresponding to transmission distances of 400 km down to 25 km for 10 Gbit/s systems. As optical networks evolve, it is highly desirable to transport data over long distance. For example, long distance transmission is crucial for four-fiber bidirectional line switched rings (BLSRS) since the protection path can be as long as the whole optical ring. Therefore, PMD compensation has become an urgent issue for high-speed optical communication systems. Although new fiber networks have better PMD performance and can support 10 Gbit/s transmission over reasonably long distance, PMD will become a limiting factor for 40 Gbit/s transmissions. As the capacity demands keep increasing, it is desirable to increase the bit-rate from 10 Gbit/s to 40 Gbit/s. Therefore, PMD compensation is very important not only for existing optical fiber networks, but also for the new optical fiber networks. However, unlike chromatic dispersion, PMD is statistical in nature, which makes it extremely challenging to compensate for waveform distortion caused by PMD.
For a laser source with narrow bandwidth, there will be two polarization modes for a single mode fiber. There is a group delay between these two eigen-modes, also known as the principal states of polarization (PSP). If the input polarization is aligned with one of the PSPs, then the output polarization will remain in the same PSP. In other words, there will be no waveform distortion if the input polarization is lined up with one of the PSPs. However, for arbitrary input polarizations, the output will consist of both PSPs with a certain amount of group delay between them. It is this differential group delay (DGD) that causes waveform distortion. In order to compensate for PMD, it is necessary to find the PSPs at the output so that a polarization splitter can be used to separate the two PSPS.
In the prior art, there are three categories of techniques are used for PMD compensations. They are: (a) all-optical, (b) all electrical, and (c) hybrid. For all-optical PMD compensation, the restoration of PMD distortion is done optically without any optical-electrical conversion. The signal remains in the optical domain. Normally, all-optical PMD compensators consist of a polarization controller, a pair of polarization beam splitters (PBSs), and either a continuous delay line or a discrete delay line such as a piece of high-birefringence optical fiber. The basic concept is to find the PSPs and align their axes to those of the PBSs. A PMD detection mechanism is then used to measure the Differential Group Delay (DGD) as the feedback signal, which is used to adjust the delay line so that the DGD is reduced to minimum. There are several different ways of measuring DGD.
In the article entitled "Polarization Mode Dispersion: Fundamentals and Impact on Optical Communication Systems" by F. Heismann, European Conference of Optical Communication (ECOC'98), Vol. 2, pages 51-79, (1998), high-speed electronics are used to measure the electrical spectrum content at specific frequencies, and then the spectral information is correlated with the DGD value. In the article entitled "Fiber-Based Distributed PMD Compensation at 20 GB/S" by R. Neo et al., European Conference of Optical Communication (ECOC'98), Vol. 3, pages 157-159, (1998), there is disclosed 77 ps Polarization Mode Dispersion (PMD) compensation for a transmission system at a speed of 20 Gbit/s using an improved RF spectrum analysis. In the article entitled "Electronic equalization of fiber PMD-induced distortion at 10 Gbit/s" by H. Bulow et al, Optical Fiber Communication (OFC'98), pages 151-152, (1998), there is demonstrated that 90 ps DGD can be compensated for by using an all-electrical method for a 10 Gbit/s system. In the all-electrical method, the distorted optical signal is converted to electrical signal at the receiver. A delay line filter with specific weights is used to partially compensate for the distortion due to PMD.
Hybrid PMD compensation is a technique that uses both optical and electrical methods to restore the distortion due to PMD. In the article entitled "Polarization Mode Dispersion Compensation by Phase Diversity Detection" by B. W. Hakki, IEEE Photonics Technology Letters, Vol. 9, No. 1, pages 121-123, January 1997, a hybrid PMD compensation technique is disclosed wherein a polarization controller (PC) and a polarization beam splitter (PBS) are used to transform the states of polarization, and split the polarization components. At each output of the PBS, a high-speed photo-detector converts the optical signal to electrical signal. An electrical delay line is used to adjust the phase delay between the two electrical signals.
There are both advantages and disadvantages for each of above mentioned techniques. For the all-optical PMD measurement technique, the usage of an optical delay line, as well as the usually complicated optical PMD measurement result in high insertion loss, and more importantly, slow compensation speed. On the other hand, the usage of a mechanical delay line raises a question of reliability. The requirement of a PMD measurement makes the compensation process relatively slow. The statistical nature of PMD also makes a high accuracy PMD measurement very difficult. On the other hand, the physical size requirement does not allow the usage of a fully featured PMD measuring device.
The electronic PMD measurement technique, using RF spectral information, suffers from laser chirp induced RF spectrum distortion, as well as distortion induced by optical fiber nonlinearity. As for all-electrical method, the finite number of delay lines makes this kind of PMD compensator good only for some specific values of Differential Group Delay (DGD). The compensation is normally partial. It evolves a high-speed electronics design, which complicates the functionality of receiver. The hybrid method also requires expensive high-speed electronics, as well as a pair of high-speed optical detectors. Both electronic and hybrid solutions are bit-rate dependent, as well as transmission format dependent.
It is desirable to provide a polarization mode dispersion compensation arrangement which (a) provides a simple optical design including a fast digital signal processing technique with low insertion loss and high compensation speed, (b) is wavelength and bit-rate independent and has no limitation on the compensation range for PMD values, (c) provides noise reduction, (d) has no mechanical moving part, and (e) is transmission format independent.